To be honest with you- The biggest difference between onpage seo and Offpage seo is that

Onpage requires brains.
Onpage is not a donkey work.
Onpage is about how good you are at analysis and research.

Offpage on the other hand is more of a donkey work.
Where you are only thinking about building links.
Daily submissions - This is crazy, although!
Offpage SEO also requires the person to be good at the writing skills and communicative one.

On-Page SEO - It is the process to make your website to optimize and user-friendly by changing the internal process of the website. It totally depends on your mind process and time-consuming. It means to check the ability of your work and thickness of your minds, how to analyze and research for optimizing the website in short time.

Off-Page SEO - It is the process to make your website more visible and popular online on the search engine. It means an external process of a website to make visible among global users in a short time. Because of Off-Page refers to the ranking factor that occurs off your website like backlinks from other sites.

2. Off Page SEO - This is related to promoting or linking your website to other reputed websites that has same relevant niche so that it impacts your website ranking on search engines. This is also called as "Link Building".